Bacteria are present in the normal gut (intestines), especially in the lower parts of the intestine. They achieve concentrations of several billion in the colon (large intestine/bowel).

These "normal" bacteria have important functions in life, including:

Protecting against infection by disease-causing bacteria

Helping the immune system of the gut to develop

Producing a variety of substances that have an important nutritional value

Together, the normal intestinal bacteria are often referred to as the gut flora (or microbiota). A number of factors may disturb the mutually beneficial relationship between the flora and the body. When this happens, bacteria that can cause disease may take hold.

What is the Role of Gut Microbiota?

There are many exciting developments and discoveries being made about the connection between our gut (digestive system) and our brain and how it affects our health, as well as the role microbes play in this connection.

It is important to know that functional GI disorders, like IBS, are not psychological diseases; but we also have to realize that the brain plays an important role. The brain communicates constantly with the microorganisms inside each of us.

These microbes have a cooperative relationship with us, not only playing a role in digestion, but also interacting and communicating with our own cells, with our gut and our brain. Gut microbes may provide a link to helping maintain a balance between the brain and the gut.

Do Bacteria Play a Role in IBS?

There is now evidence to support the idea that disturbances in the bacteria that populate the intestine may have a role in at least some people with irritable bowel syndrome (IBS).

This evidence from observations or studies can be summarized as follows:

Antibiotic use, well known to disturb the flora, may predispose individuals to IBS

Some people may develop IBS suddenly following an episode of stomach or intestinal infection (gastroenteritis) caused by bacteria (a condition called post-infectious IBS or PI-IBS)

A very low level of inflammation may be present in the bowel wall of some IBS patients, which could have resulted from an abnormal interaction with bacteria in the gut

Small intestinal bacterial overgrowth (SIBO) may be associated with IBS

Altering the bacteria in the gut, by antibiotics or probiotics, may improve symptoms in IBS

In PI-IBS some people who were previously well develop IBS-type symptoms following an episode of gastroenteritis, while most others recover completely. PI-IBS represents a clear link between exposure to a bacterial infection (such as from contaminated food or water) and IBS in those who seem especially at risk.

A number of research studies report a role for inflammation in the bowel’s inner lining (mucosa) in IBS. This is low-grade inflammation and far less than that seen in the true inflammatory bowel diseases, ulcerative colitis and Crohn’s disease. Interestingly, it has been found among patients in whom there was nothing to suggest that their IBS began with an infection. What suddenly causes this inflammation is not clear. It may be that subtle changes in the bacterial population in the intestines are driving it.

Some people with IBS may be genetically predisposed to an exaggerated inflammatory response to normal bacteria. While this theory of IBS is in its infancy, there is already some evidence for the extension of the inflammatory process beyond the confines of the gut wall. This could explain some of the symptoms such as tiredness and fibromyalgia that may occur in IBS sufferers.

For some time, various studies have suggested the presence of changes in the kind of colonic flora in people with IBS. These changes in the flora could lead to the increase of certain bacterial species, which themselves produce more gas and other products of their metabolism. This could contribute to symptoms such as gas, bloating, and diarrhea.

Is there a Role for Antibiotics in Treating IBS?

Normally, bacteria are virtually absent in the acid environment of the stomach and are few in the upper gut. They increase considerably in the lower parts of the small intestine, and then dramatically on crossing into the colon.

Recently, it has been suggested that the small intestines of some IBS patients have increased numbers of bacteria as well as types of bacteria normally found only in the colon. This condition is called small intestinal bacterial overgrowth (SIBO). A course of antibiotics may improve symptoms, but experts are divided on whether or not SIBO plays a role in IBS.

The question remains as to why at least some people with IBS respond to a course of antibiotics. This has most recently been reported in studies of IBS patients treated with the antibiotic rifaximin.

Another explanation for these responses may be the suppression of certain species of bacteria in the colon, and especially those bacteria that are prone to produce gas and other substances through fermentation. An improvement of gas-related symptoms, with antibiotic therapy, has been described among a group of patients with bloating and flatulence, who did not have evidence of SIBO. Antibiotics clearly help some IBS patients though how this happens may be more complex than originally thought.

A prolonged course of antibiotic therapy should be approached with caution, regardless of the safety profile of a given antibiotic. This is an important issue, as IBS tends to be a chronic and relapsing condition. Some people obtain a relatively long-lasting improvement following a single course of antibiotic, but others do not. Results of further long-term studies regarding the use of antibiotics are needed.

Is there a Role for Probiotics in Treating IBS?

Given their safety profile, effective probiotics would, at first sight, appear to be more attractive as a means to influence the gut flora in IBS. Are probiotics effective in IBS? There have been several studies of a variety of probiotics in IBS. It is only recently that these have been of the quality needed to come to firm conclusions. There was some early evidence, although inconsistent, of symptom improvement in "gas-related" symptoms.

More recently, there has been some evidence of benefit from some probiotic combinations. The best evidence relates to a particular organism, Bifidobacterium infantis 35624. Studies have shown this strain to be superior to placebo in relieving the main symptoms of IBS (abdominal pain/discomfort, difficult defecation, and distension/bloating).

Summary

Many recent findings add to a growing body of evidence to suggest that IBS may result, at least in part, from a dysfunctional interaction between our gut flora and ourselves. This leads to a low-grade inflammation in the gut wall that may spill over into other areas of the body.

Some researchers propose a role for bacterial overgrowth in the small intestine as a common factor in causing the symptoms of IBS. Other evidence points to more subtle changes in the colonic flora. Both hypotheses remain unconfirmed.

Nevertheless, short-term therapy with either antibiotics or probiotics does seem to reduce symptoms among some people with IBS. Whether or not these effects come about through changes in the number or type of bacteria in the colon and/or small intestine, it is evident that manipulation of the gut bacteria deserves further attention.

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